Question

I need a shared_ptr like object, but which automatically creates a real object when I try to access its members.

For example, I have:

class Box
{
public:
    unsigned int width;
    unsigned int height;
    Box(): width(50), height(100){}
};

std::vector< lazy<Box> > boxes;
boxes.resize(100);

// at this point boxes contain no any real Box object.
// But when I try to access box number 50, for example,
// it will be created.

std::cout << boxes[49].width;

// now vector contains one real box and 99 lazy boxes.

Is there some implementation, or I should to write my own?

Answer 1

It’s very little effort to roll your own.

template<typename T>
class lazy {
public:
    lazy() : child(0) {}
    ~lazy() { delete child; }
    T &operator*() {
        if (!child) child = new T;
        return *child;
    }
    // might dereference NULL pointer if unset...
    // but if this is const, what else can be done?
    const T &operator*() const { return *child; }
    T *operator->() { return &**this; }
    const T *operator->() const { return &**this; }
private:
    T *child;
};

// ...

cout << boxes[49]->width;